This invention relates to a magnetron, more particularly, a magnetic circuit of a magnetron of the type containing permanent magnets in its evacuated container.
When incorporated into a microwave oven, a magnetron generates a microwave output with high efficiencies and is therefore used widely for defrosting and heating foodstuffs.
While a prior art magnetron typically includes permanent magnets located on the outside of an evacuated container, a magnetron of the other type has also been proposed in which permanent magnets are contained in an evacuated container for the purpose of reducing size, weight and cost of manufacturing.
The prior art of the latter type has however disadvantages as described below. During the operation of the magnetron, a microwave is generated in the magnetron tube, which increases especially when the load is reflective. For this reason, should the microwave prevail in gaps between component elements or at portions where the joints between the elements are not perfect, electric sparks may occur, resulting in local heating and gas generation. As a consequence, the degree of vacuum in the magnetron tube is decreased, thus causing it to operate abnormally. This phenomenon occurs especially between the upper permanent magnet and its pole piece on the output (or antenna) side of the magnetron. Usually, the pole piece has a top surface overall area of which is in direct contact with the bottom surface of the upper permanent magnet for the purpose of establishing uniform field efficiently in the interaction space. To assure perfect contact with the magnet, the top surface or contact surface of the pole piece should be a flat surface of high flatness. Of course, no foreign matter should be present between the contacting surfaces. However, the construction of the prior art pole piece is difficult to meet hight flatness and prone to intimacy with foreign matters.